7.1 Introduction
The viscoelasticity of a compound is intimately related to its processability in the postmixing
operation such as extrusion and injection moulding. In this chapter, however,
viscoelasticity is used to characterise the end-product of mixing. One area of importance
is to relate the viscoelastic properties to the composition of the compounds. Because the
rubber formulation usually contains many additives, only the additives which have a
significant influence on mixing and on viscoelastic properties of the compound are
discussed. This narrows down the material variables to rubber grades and fillers. The
low MW additives such as extending oils, plasticisers and tackifiers are important but
are not included here except for an example of the oil-extended rubber. All the compounds
were mixed to give a satisfactory degree of homogeneity, judged to be acceptable on the
basis of commercial standards.
7.2 Viscoelastic properties of compounds
The mechanical properties pertinent to the mixing of rubber include viscoelastic properties
at small as well as large deformation and failure behaviour. An overriding question is
how the viscoelastic properties of gum rubber contribute to these properties of the
compounds. This question is answered by choosing several grades of gum rubbers which
have the same chemical composition but are different in molecular architecture and
therefore, have different viscoelastic properties. Selecting a standard formulation which
gives satisfactory mixing, the viscoelastic properties of gum rubbers and compounds
may be compared. Samples for this experiment were NBRs with 33% acrylonitrile content
[1] (see Table 7.1).
The characterisation of gum rubber is discussed extensively in Chapter 6 and reference
[2]. Samples A and B are gel-free if analysed using ASTM D3616-95 [3]. The main
difference is in the Mooney index [4]. Samples B, C, and D have similar Mooney values
but their viscoelastic properties are very different from each other. Sample C contains
50% microgel, crosslinked particles, whereas Sample D has 75% macrogel, a molecule
having many long branches. The compounds contain 40 phr N550 carbon black
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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